Thin multi-chip flex module

ABSTRACT

A flexible circuit comprises a folded dielectric sheet having conductive patterns on its surface(s) to which microelectronic device(s) are attached. The dielectric sheet is folded 180° about a selected axis and a bond layer joins the two halves over a portion of their respective surface areas so that a remaining portion of their areas remain unbonded and a bifurcated structure is thereby formed. Electrical contacts are provided on the unbonded or bifurcated portions of the flexible sheets. The flex may be attached to a rigid frame and provided with protective heat spreading covers. The folded flex design is particularly suitable for reel-to-reel manufacturing.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Provisional Patent ApplicationNo. 61/009,781 by the present inventors, filed on Jan. 2, 2008, theentire disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to means for packagingmicroelectronic devices and multichip modules. More particularly, theinvention relates to improved flex-circuit solutions for SIMM and DIMMtype memory modules and other modular electronic circuits.

2. Description of Related Art

High density computing is continually evolving to offer high performancedevices with higher densities and smaller form factors. Servers aredesigned with these goals in mind and are key enablers for variouscomputing applications. Servers contain, among other electroniccomponents, microprocessors and memory modules, which generateconsiderable heat during operation. Currently higher performance serversmake use of dual and quad core microprocessors. In turn, thesemicroprocessors require memory modules with greater memory capacity foroptimal performance.

Another important aspect of server design is that servers are oftenarranged in closely packed groups set in vertical racks that rely onforced air cooling for heat exhaust. So, air cooling is done for eachserver unit as well as the entire rack of server units. When multiplememory modules are tightly spaced together inside the tightly packedserver chassis, the circulation of air as a cooling fluid becomesineffective. Air can be restricted, leading to overheating of memorymodules which in turn leads to premature failures including devicefailures and corrupt data streams.

The desirable attributes of the best memory modules include: high memorydensity, reduced thickness, reduced height, good thermal exhaust, goodsignal integrity, reliability, manufacturability and reduced cost. Thesecharacteristics are inter-related and the optimization of one mayadversely impact the other; so, a balance must be found among thesevarious characteristics to determine the most effective solution. Themost effective solution may be relative to a given memory density. Inother words, the most effective memory module solution at 2 GB or 4 GBmay be different than the solution for 8 GB and 16 GB modules.Furthermore, the best DIMM solution for DDR2 DRAMs may be different thanthe best DIMM solution for DDR3 and DDR4 DRAMs since the standards areslightly different.

Increasing memory density inside a module can be achieved throughstacking of chips. However, stacking of chips is accompanied by anincrease in cost of the stacked chips. Also, stacking of chips isaccompanied by greater heat that is concentrated between the stackedchips and is more difficult to dissipate which leads to an overallhotter module. Additional heat exhaust can be obtained through the useof heat spreaders or heat sinks. However, the addition of larger heatsinks and heat spreaders is accompanied by an increase in thickness. Inturn, increasing thickness leads to a pressure drop between adjacentmodules which impedes the flow of air and adversely affects cooling. Thewords heat spreaders and heat sinks are used interchangeably in thisinvention.

Thickness reduction can be achieved through the use of thin laminatesand flexible circuits. However, flexible circuits can be more expensivethan standard PCB based on FR4 materials. Furthermore, thicknessreduction causes a mismatch between the thin laminate or the flexiblesubstrate and the standard connector.

As previously taught by J. E. Clayton in a series of patents detailingthe use of flexible circuits for memory modules (U.S. Pat. No.6,665,190, U.S. Pat. No. 6,232,659, U.S. Pat. No. 6,091,145, U.S. Pat.No. 6,049,975, U.S. Pat. No. 5,731,633, U.S. Pat. No. 5,751,553, U.S.Pat. No. 5,708,297, U.S. Pat. No. 5,661,339, 2007/0211426A1,2007/0212902A1, 2007/0212919A1, 2007/021171A1, 2007/0212906A1,2007/0212920A1) the design for the optimal thermal path puts the chip ina configuration that optimizes the heat path between the heat generationsource and the metallic heat sinks and heat spreaders. However, when thechips are in an optimal heat path configuration, they may not be in anoptimal electrical path configuration.

Flexible circuits exhibit many desirable attributes that lend themselvesto solving many electronic packaging problems. Because they areconstructed using thin flexible laminates, flexible circuits can beadapted into a large variety of three-dimensional configurations. Inparticular they are uniquely suited for joining two separate circuitcomponents that involve repeated dynamic flexing motions such as whenopening and closing cell phone, camera and notebook LCD displays.

Flexible circuits are also used in applications where reduced thicknessor curved surfaces are important. Many mobile products produced todayare made feasible by the unique characteristics inherent with flexiblecircuits. For background purposes, a fairly comprehensive description offlexible circuit technology, including construction methods, design andapplication specific examples may be found in a book authored by Dr.Joseph Fjelstad entitled “Flexible Circuit Technology” (3^(rd)Edition—September 2006) the teaching of which is incorporated herein byreference in its entirety. Another reference on flexible circuits is“Coombs' Printed Circuits Handbook—Fifth Edition” by Clyde F. Coombs,Jr., the teaching of which is incorporated herein by reference in itsentirety. Lastly, “Foldable Flex and Thinned Silicon Multichip PackagingTechnology” edited by John W. Balde, is incorporated herein by referencein its entirety.

High performance computers, such as server and super computers, involvemany dense, high frequency electrical connections where flexiblecircuits may be advantageously employed. Their thin uniform laminatethickness, ability to be fashioned with fine lined traces and small viasfor layer-to-layer interconnections are better suited for higherfrequency operation than traditional rigid printed wiring boards (PWB).

As clock frequencies increase with each succeeding generation ofmicroprocessors, there is an increasing need to design circuitmotherboards using techniques for controlled impedance and signalintegrity. Usually this results in an increase in the layer count ofrigid PWB circuit boards. Computer motherboards that could previously bedesigned with only 4 or 6 laminate layers now require 8 or more layersto properly route traces operating at higher clock rates. This mayincrease the cost of these special PCBs.

Using finer wiring patterns, flexible circuits can significantly reducethe number of required layers to form the same circuit functions. RigidPWB motherboards are, nevertheless, presently required for mounting manyhardware pieces such as power supplies, disc drives, fans, and componentsockets and are therefore in no danger of being eliminated in theforeseeable future. However, the advantages of flexible circuits, asnoted above, are leading many engineers to look for creative ways inwhich to include them in their designs. Examples developed bySiliconPipe are described by co-founder Dr. Fjelstad and illustrated onpages 32 and 33 of the reference text “Flexible Circuit Technology”cited above.

Although the use of flexible circuits in packaging semiconductors andconsumer electronics is well known and offers the capability of highdensity interconnect signal stability at high frequencies, and flexibleform factor (connecting sites that are not aligned), the technologybased on flexile circuits is not without disadvantages. Flexiblecircuits present an overall cost disadvantage, they are inherently anon-rigid form factor and need structural members to be incorporated.Furthermore, the technology needs special design talent for highperformance, reliability and operation at high frequencies. One of thechallenges caused by the use of flexible circuits is that it reduces thethickness of the substrate to the point that it becomes incompatiblewith current standard connectors.

Multi Chip Modules (MCM) have a known form factor with known advantagesand disadvantages. The advantages of thickness reduction in DIMMapplications are increase air flow and space savings on the motherboard. However, solving the thickness issues create other issues.Thickness reduction can be achieved by the use of Thin PCB, Rigid Flex,Flex circuitry (connected to standard PCB connector), or by using Flexcircuit exclusively.

The thickness of a PCB can be reduced as is the practice of companiessuch as Eastern Company. These thin laminates can reduce thickness ofthe PCB but may not be good for high density modules and high frequencyoperations due to the limited number of layers they utilize and theirdielectric properties. However, when thin PCB laminates are used atransition between the thin laminate and the wider DIMM connector isneeded.

Objects and Advantages

It will be appreciated that the most effective module solution may notnecessarily be the best in any one particular performance attribute, butrather offers the best overall solution at a given density and DRAMclock speed. Objects of the present invention include at least thefollowing: providing effective solutions for thickness reduction inelectronic modules; providing electronic modules that are thinner andmore cost effective; providing electronic modules suitable for placementat higher densities; providing improved memory modules for computingapplications; providing improved memory modules for blade servers; and,providing a method for manufacturing electronic modules that is costeffective and allows for rework.

Additional objects of the invention include providing a means forefficient thermal communication between the enclosed integrated circuitdevices and the exterior surfaces of the thermally conductive shell inan electronic module; maintaining the shortest thermal path from thesurfaces of the enclosed chips to the ambient air outside an electronicmodule; adapting a portion of an interconnecting thin flexible circuitor thin printed circuit board to provide exterior contact pads forelectrical and mechanical mating with existing DIMM type sockets;adapting electronic modules with stackable connections whereby modulesmay be stacked in vertical and/or lateral configurations; and, providingan improved means for locating passive components in circuit modules.These and other objects and advantages will become apparent from readingthe specification in conjunction with the accompanying drawings, whichare not necessarily drawn to scale.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a method for making amultichip module comprises the steps of: forming a continuous dielectricsheet, the sheet having a conductive pattern on a first surface thereof;attaching a plurality of microelectronic devices to the first surface sothat the microelectronic devices are operably coupled to the conductivepattern; applying a bonding material to a second surface of thedielectric sheet; folding the sheet so that the microelectronic devicesare on the outer surface of the folded structure; and, bonding the innersurface of the folded sheet over at least a portion of its area usingthe bonding material.

In accordance with another aspect of the invention, a multichip modulecomprises: a flexible dielectric sheet having a conductive pattern on afirst surface thereof and a bonding layer on at least a portion of asecond surface thereof; a plurality of microelectronic devices disposedon the first surface so that the microelectronic devices are operablycoupled to the conductive pattern; a 180° fold about a selected axis inthe dielectric sheet so that a first portion of the second surface isbrought into contact with a second portion of the second surface andbonded thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically several ways in which a bifurcated flexaccording to the invention can be used to make an up-transition betweena thin circuit and a wider socket.

FIG. 2 illustrates schematically a bifurcated flex according to thepresent invention in which two metallized flex layers are bonded over aportion of their respective areas and not bonded over another portion.

FIG. 3 illustrates schematically the prior art in which a single flex iswrapped around a supporting member, with electrodes on the outersurface(s).

FIG. 4 illustrates schematically a bifurcated flex in which the free(unbonded) portions are reflexed and provided with electrodes inaccordance with one aspect of the invention.

FIG. 5 illustrates schematically in cross section a flex module inaccordance with one aspect of the invention.

FIG. 6 illustrates schematically in cross section another flex module inaccordance with one aspect of the invention, illustrating an alternativesequence of construction.

FIGS. 7-10 illustrate schematically in cross section some other ways ofconstructing the flex module of the present invention.

FIG. 11 illustrates a stiffener with hollowed-out areas in accordancewith one aspect of the invention.

FIG. 12 illustrates schematically various configurations of a protectivemetal frame in accordance with several aspects of the invention.

FIG. 13 illustrates schematically in cross section a module withelectrodes disposed on two opposite edges in accordance with one aspectof the invention.

FIGS. 14-15 illustrate schematically some alternative ways of joiningseveral flex modules together to expand functionality or memory capacityin accordance with one aspect of the invention.

FIG. 16 illustrates schematically an arrangement in which a bifurcatedflex is disposed within a protective frame that has spacers to protectthe semiconductor devices from crushing in accordance with one aspect ofthe invention.

FIG. 17 illustrates schematically a flex module in which the bifurcatedflex is disposed within a molded plastic frame in accordance with oneaspect of the invention.

FIGS. 18-20 illustrate schematically various arrangements of abifurcated flex according to the present invention, including bothsymmetrical and asymmetrical configurations.

FIGS. 21-22 illustrate schematically an arrangement in which moduleshave more than one bifurcated flex arranged to engage a second module ora corresponding socket in accordance with one aspect of the invention.

FIG. 23 illustrates schematically one way in which embedded passives maybe incorporated into the invention.

FIGS. 24-34 illustrate schematically various arrangements and featuresthat can be incorporated into flex modules in accordance with thepresent invention.

FIGS. 35-36 illustrate schematically a foldable frame configured tocontain a flex circuit according to the present invention.

FIGS. 37-38 illustrate schematically two embodiments of the invention inwhich supporting frames are held together by mechanical screws and snapsrespectively.

FIGS. 39-43 illustrate schematically various arrangements and featuresthat can be incorporated into flex modules in accordance with thepresent invention.

FIG. 44 illustrates schematically a multichip module constructed usingfolded flex compatible with reel-to-reel processing according to oneaspect of the invention.

FIGS. 45-46 illustrate schematically two multichip modules according tothe invention.

FIG. 47 illustrates schematically a multichip module in which the frameand covers form a clamshell arrangement according to another aspect ofthe invention.

FIG. 48 illustrates schematically a multichip module having a centralheat pipe, a clamshell type cover, and an external heat exchangeraccording to another aspect of the invention.

FIGS. 49-52 illustrate schematically several designs for multichipmodules in accordance with various aspects of the present invention.

FIG. 53 illustrates schematically a multichip module havingcastellations on the external contact pads according to another aspectof the invention.

FIG. 54 illustrates schematically in cross section a multichip moduleaccording to one aspect of the invention, which is suitable forattaching to a motherboard by soldering to plated through-holes.

FIG. 55 illustrates schematically a low insertion force socketarrangement according to another aspect of the invention.

FIG. 56 illustrates schematically a low-profile pin contact arrangementsuitable for use with castellated contact pads according to anotheraspect of the invention.

FIG. 57 illustrates schematically the incorporation of posts or spacersin the module to protect the semiconductor devices from stressesaccording to another aspect of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention addresses the designs of effective solutions tooptimize the offering and differentiation for thickness reduction,module densities and relative cost effectiveness. These parameters arebalanced per density requirement. For instance, a design that can meetand optimize the attributes of a memory module at the low densities (2GB and 4 GB) can use a central flexible circuit that places the chips ina mirror image configuration and that offers the benefit of using alesser amount of flexible circuit and hence enables lower cost. Chips ina mirror image around a flex enable simpler electrical design.Furthermore, the centrally-located flex reduces thickness and whencoupled with other features in the current invention (including thosefeatures dealing with signal length symmetry, grounding strategies, andthe connection interface), would produce an extremely thin andrelatively cost effective solution at the low densities.

Skilled artisans will appreciate the value of reducing the thickness ofan electronic module. Reducing the module thickness allows cooling airto flow more freely between the modules and to pass through and overother components behind the modules that would otherwise be “shadowed”from the cooling fluid. The thickness of the memory module is presentlydictated by the thickness of the PWB substrate, the thickness of thememory packages mounted on the substrate's surfaces, and more recentlythe additional thickness imposed by bulky heat sinks. These heat sinksare typically clipped over both sides of the module.

Reducing IC packaging thickness is a continuing trend, but appears tohave reached a practical limit. Another solution presently beingdeveloped is to stack multiple memory devices within the same packagedimensions. However, assembly yield, and hence cost, are adverselyeffected, and stacked memory devices tend to cause more heat to begenerated and concentrated between the memory devices themselves.

A solution is provided in this invention by substantially reducing thesubstrate thickness, allowing the memory components to be positionedcloser to the centerline of the module, and incorporating the heat sink16 (herein referred to as a heat spreader or exterior metal shell ormetallic enclosure or stiffener plate(s)) as an integral part of themodule to compensate for the reduced rigidity of the thinner substrate.The metal walls in the metal shell serve multiple purposes (chipprotection, heat spreading, connector, stiffening, interface, groundingand assembly). In addition to enabling a significant reduction inoverall thickness, this invention also provides a robust yet lightweightenclosure for protecting minimally packaged and/or unpackaged, “bare”silicon memory devices that otherwise would be exposed to handlingdamage if assembled on the exterior surfaces of conventional DIMMsubstrates. The standard IC devices are typically individuallypre-packaged devices (e.g. FBGA, μBGA, CSP, etc.), but the use ofunpackaged bare silicon chips (e.g. wafer-level RDL, flip chip, DCA,etc.) would be preferred if its use is made possible and practical.Wafer Level Packaging of semiconductors can achieve very small and costeffective semis packaging form factors. Bare silicon memory devicesrepresent the ultimate minimalist design or thinnest form factor sincethey are not surrounded with conventional molding compounds that aretypically used to house and protect IC components. But the industry hasgrown dependant on pre-packaged components as a means for handling andtesting memory devices to various grade levels.

Recent advancements in wafer probing and testing technology may soonenable the long sought goal of a reliable, high volume supply of KnownGood Die (KGD) to the industry. The present invention enables theconstruction of a standard memory module specifically designed toenclose and protect these fragile KDG memory devices in order for thetechnology to advance. The package-less die leads to savings at the chiplevel. This saving per unit silicon is multiplied by the number ofplacements at the module level which leads to significant savings whenthe module has 18 placements, 36 placements, 72 or 144 placements.

It will be appreciated that although the present invention represents asignificant departure from prior electronic module manufacturingpractice, it is clearly sufficiently “manufacturable” to becomequalified through recognized industry standardization processes,particularly for memory modules. The invention can enable memory devicemanufacturers and third party module manufacturers to assemble finishedmodule products directly from memory wafers without the necessity ofpre-packaging each individual die first, resulting in a considerablesavings in cost and process steps. This is a significant cost reductionfor DRAM manufacturers compared to the existing art. By reducing costand assembly time for DRAM manufacturers, the invention is expected tobe a significant enabler of a revolutionary “package it once” trendwithin the industry Furthermore, significant cost savings can beachieved by the DRAM manufacturers from an operational stand point byeliminating the packaging operations within the organization and byaligning themselves closer to a Just-In-Time business model.

Another advantage of the invention is to significantly reduce themounted height of the module. Reducing the mounted height allows thesemodules to either fit into products that are very restricted in height,such as blade servers and mobile devices, or to add more memory insidethe module as a result of a net gain in component space. The reductionin module height is achieved by means of a unique contact architectureenabled by this invention that allows the seating plane to be reduced inthe DIMM socket, as detailed below.

The direct module attachment to mother boards enables the reduction inparasitic signals and high bus speed through reduction of contacts andtransitions. Some of the module interfaces described in this inventionare applicable to high speed and large bandwidth electronics. Thebenefit at the mother board level is the reduction of large holes in themotherboard that may inflict speed penalties. By eliminating thestandard connectors that are soldered at the mother board level, and bychanging the signal routing at the motherboard level, we enable theoptimization of high frequency designs that are more stable and scalablewith future requirements. It recognizes that the designs of the novelmodule designs are coupled or done in conjunction with signal routinglayout changes at the mother board level. By adopting flexible circuitsat the module level, we also enable better signal integrity, widervoltage margins and more thermal stability compared to module on circuitboard alternatives.

The present inventions can be expanded beyond memory modules. In somecases the ICs residing in the modules described in the presentinventions can be a combination of logic and memory devices. Certainaspects of the present invention teach and describe the concepts ofTotal System in Module (TSIM) or the “brain”, whereby memory andmicroprocessors reside in one very thin module/card suitable for themobile market. The up-grade of a mobile device may then be done throughthe replacement of the thin module containing the TSIM.

Another advantage of the invention is to achieve higher density throughstacking at the module level not just at the chip level. In either thestandard height version or the reduced module height version with zeroseating plane, the connection between two modules is engineered toenable the stacking and entering into a functional state of electricalcontinuity when inserted (or engaged) into one another.

The solution to the desirable target thickness reduction and the otherdesirable characteristics can be achieved by two distinctly fundamentalways. In one solution the thickness advantage is achieved and theconnector interface is built using a standard PCB. The second solutionachieves the goal using an alternate connection method through the useof bifurcated flex as the interface with the standard connectors. Boththese innovative aspects are addressed in the current disclosure.

One of the desirable attributes of this invention is to achieve amultichip module of minimum thickness for enclosing integrated circuit(IC) devices. In order to achieve minimum thickness, these devices arepreferably interconnected by a thin flexible circuit or thin printedcircuit board placed at the center vertical axis of the module. The ICdevices can be individually pre-packaged devices (e.g. FBGA, μBGA, CSP,etc.), but are preferably unpackaged bare silicon chips (e.g.wafer-level RDL, flip chip, DCA, etc.) in order to achieve minimumthickness of the module and reduce cost per chip unit as well as per thenumber of placements on the module.

Another desirable attribute is to provide a rigid, integral, thermallyconductive outer shell which forms an interior cavity within the moduleto protect the integrated circuit devices and interconnecting substratecontained within. It will be appreciated that this outer shell may havemulti-purpose functionality. The outer shell can: a) serve electricalfunctionality for extending the module grounding plane; b) serve as theassembly carrier for the module; and c) be shaped to mate with theconnector.

According to one aspect of the present invention, card edge connectorsare fashioned using bifurcated sections of flexible circuits, intendedfor insertion into a socket. A flexible circuit is sandwiched in themiddle between two adjacent substantially rigid stiffener plates 16 withbifurcated flex circuit sections extending some distance along the outersurfaces of the stiffener plates. The stiffener plates can either beflush at their respective edges or staggered, FIG. 17. The bifurcatedsections of the flex circuits contain socket contact pads disposed ontheir exposed surfaces when folded back upon the edge(s) of thestiffener plate(s) and bonded into position, or when extended parallelto the exposed surface of a staggered stiffener plate and bonded inplace, FIGS. 18, 19. Sockets intended to engage with the variousembodiments of these inventions may be conventional DIMM type sockets 50or they may be fashioned from similar arrangements of bifurcatedflexible circuits with pads containing raised metal bumps, FIGS. 20, 21.The flexible circuit sections of the mating sockets are likewise bondedto stiffener plates in a manner that enables the card edge connector,whether male or female, to securely engage with its respective socket,whether male or female. Raised metal bumps may be disposed on theflexible circuit pads of either the card edge connectors or matingsocket or both. Alternatively, owing to the compliant nature of the flexmaterial, raised metal bumps on the surface of the flex pads may beformed using protruding structures that are formed on the surfaces ofthe stiffener plates and which are positioned to lay beneath the bondedflex circuit section pads.

One aspect of the invention involves the use of a bifurcated flex toprovide the connecting transition between a thin laminate to a standardDIMM connector 50. This transition is called an Up-transition to reflectgoing from thin to thick. A standard DIMM PCB substrate 51 thickness for5V operation is 1.27 mm, FIG. 1A. The standard PCBs have lateral padsthat mate with the pins inside a standard DIMM Connector, FIG. 1.However, when a thin laminate PCB 52 is substituted it will not properlymate with a standard DIMM connector across its width, FIG. 1B. Asolution to bridging a thin laminate circuit to a larger width connectoris achieved through a flex circuit transition 55, FIG. 1C, across thewidth and length of the module. Since the transition in this case ismade from thin to thick, it is called an up transition, FIG. 1D.

It will be appreciated by skilled artisans that the flex portions usedfor an up transition or a down transition are made with impedancematching capability between the two portions to be connected or engaged.

It is conceivable that high frequency designs may require an increase inthe number of layers in the PCB 53, FIG. 1E. A down transition 56 istherefore contemplated, whereby a flexible circuit mates with each padto reduce the effective thickness at the connector portion, FIGS. 1F &1G. Thin PCB laminates 52 reduce the thickness, FIG. 1B, but not as muchas flexible circuits, FIG. 1H. Flexible circuits, however, need atransition 57 to mate with standard connectors as well, FIG. 1J. Asolution similar to the one described for thin laminates can be used,FIG. 1K. However, the present invention provides a better solution inthe form of a bifurcated flex where the adhesive in the center of thelaminate is discontinuous and allows the flex circuit to be parted intotwo distinct laminae that may provide a reference ground plane as wellas the appropriate signal layers.

The flex bifurcation in this invention may also achieve other purposesas will be described in the following examples.

Bifurcated Flexible Circuit:

In some applications the flexible circuit is a multilayer structure thatconsists of multiple dielectric core films with patterned copper traceson one or both surfaces of the dielectric core to form a single flexcircuit ply. Separate patterned plies of copper/dielectric sheets canthen be laminated together with various adhesives materials to form amultilayered flexible circuit. In some applications it is desirable tolimit the adhesive joining the separate flexible plies of a multilayercircuit to be discontinuous. This technique is commonly employed toallow individual plies to be separated apart and connected to differentlocations on the circuit board. For background purposes, examples ofdiscontinuous plies are illustrated in FIGS. 1-4 of page 8, the bottomfigure of page 24, FIG. 5-5 and 5-6 of page 80 and FIG. 6-20 on page 143of the reference book “Flexible Circuit Technology”.

For purposes of this application Applicants use the term “bifurcated” todistinguish between other methods for separating flexible circuits. FIG.2 illustrates a bifurcated multilayer flexible circuit 10. When adiscontinuous adhesive 2 is formed within a multilayer flex circuit, theindividual plies 4 are separable 5 beginning at a point 6 within theplane of the circuit's thickness. For example, if a four-layer flexiblecircuit, as shown in FIG. 2, is fashioned using two separate plies 4with copper patterns 8 disposed on both surfaces of each polyimide coredielectric 12 and the adhesive 2 bond that joins the separate plies atthe center-plane is discontinuous, then the two plies can be spacedapart 5 beginning at a point 6 in the center-plane where the adhesive isdiscontinuous. In the example shown the flexible circuit is “bifurcated”into two separate plies with double-sided copper films. Applicantsdistinguish a “bifurcated” flexible circuit from a “split” flexiblecircuit in which the circuit is cut through all laminate layers in avertical axis, as believed to be represented in FIG. 2-27 of page 33 inthe referenced text.

In some applications the flexible circuit is adhesively bonded to arigid stiffener such as a plastic or metal plate without any directelectrical contact with the object providing a rigid mounting surface.An example would be a thin flex circuit bonded to a metal heat spreaderfor cooling one or more chips.

FIG. 3 is an illustration of prior art showing how an edge cardconnector 20 can be fashioned by wrapping a single ply 4 in aunidirectional manner around the edge of a stiffener 16 and bonding theflexible circuit with an adhesive material 14 such that electricalcontact pads 18 are arrayed along both top and bottom edges of thestiffener. The contact pads may be electrically connected to either theouter 7 or interior 9 copper patterns 8 of the flex circuit ply usingplated via holes (not shown) cut through the dielectric core 12.

The example shown in FIG. 3 was the state of the art taught by J. E.Clayton and requires all electrical signals associated with the leftpads to be routed across a longer distance than signals associated withthe right contact pads. Extra trace length is required to bring thesignals around the edge or nose of the stiffener and past the rightcontact pads. Although the extra trace length may be inconsequential formany lower frequency applications, at high operating frequencies thisextra length may become a problem. Furthermore, at high frequenciesbetter grounding designs are needed. Therefore, designs that enable highfrequency performance as well as signal symmetry are preferred. Thecurrent invention leads to these solutions with extended groundingplanes, signal symmetry, and equidistant traces.

Signal Symmetry Through Bifurcation

An improvement over the aforedescribed prior art is illustrated in FIG.4. In this example a central bifurcated multilayer flexible circuit withexternal contact pads 11 enables the separate plies to be wrapped inopposite directions (i.e. bi-directional manner) outward and backwardsaround the ends of two stiffeners 16 that surround and enclose themultilayer flex circuit from both sides. As shown, this can be used toproduce a card edge connector using a bifurcated flex circuit 30. Inthis example the signal paths are balanced and of the same trace length.

When a chip 22 is connected to a flexible circuit, a stiffener is neededto give the assembly structural integrity, FIG. 5. The stiffener can befrom one side or from two sides. For the purpose of this invention, adouble-sided stiffener is needed. The stiffener that is on the chip sideis preferably designed with a recessed portion to allow the chip to nestinside a cavity. This cavity protects the chip and enables the use of aKGD—a package-less chip. Alternatively, the back stiffener can be solid16″. The dimensions of the stiffeners are preferably such that whencombined with the thickness of the adhesives and the thickness of theplies, the total thickness is compatible with the width of a standardconnector 50.

When chips 22 are placed in a mirror image configuration of a centralflexible circuit, the stiffener on both sides of the flex is designedwith a recessed cavity to host one or more chips (see FIG. 6A). Theoverall thickness is preferably maintained to be compatible with astandard connector 50. An added benefit in this example is that theflexible circuit is repositioned at the centerline of the sandwichedstructure, which provides better symmetry and an opportunity to create ametal enclosure around the various components mounted to both sides ofthe central multilayered flex circuit 11. As shown in FIG. 6A, adiscontinuous adhesive layer 2 may be used to join two subassembliesinto a complete module. Alternatively, the two subassemblies may bejoined at a plurality of discrete points as shown in FIG. 6B. Thesediscrete points may comprise bump contacts 24′ or pads electrically andmechanically joined with solder connections 95 or may be isotropic oranisotropic conductive adhesive 99, etc. It will be appreciated thatthese points may coincide with conductive vias (not shown) in the flexcircuits, whereby signals from one subassembly may be communicated tothe other subassembly. The resulting architecture is analogous to thatshown in a slightly different form in FIG. 44.

Electrical ground pins, disposed among the array of external contactpads 18 of the outer copper patterns 7, can bring grounded connectionsthrough plated via holes (not shown) to the interior copper patterns 9.Then, when an electrically conductive adhesive is used as the bondingmaterial 14′ for attaching the bifurcated plies 4 to metal stiffeners16, the metal stiffeners can function as a reference ground plane thatprovides protection for interior mounted IC devices from Electro-StaticDischarge (ESD) and Electro-Magnetic Interference (EMI).

The metal stiffeners also provide a rigid shell for mechanicalprotection of fragile bare-IC die 22′ or stacked die 22″ disposed on thesurfaces of the flexible circuit and a heat spreading surface to conductheat away from the enclosed IC devices. The metal shell is preferablyfashioned to maintain compatibility with standard connectors 50. Thedimensions and mechanical sturdiness required for proper operation isdesigned into the shell.

FIG. 6A illustrates an example of very thin memory module 40 constructedin a manner consistent with that described for FIG. 4. In thisillustration memory chips 22, stacked die 22′ or bare die 22″ are flipchip bonded to both surfaces of a central, multilayer, bifurcatedflexible circuit 11 using an array of bump contacts 24. The bumpcontacts may comprise solder balls 93 or Ni—Au plated bumps incombination with isotropic conductive adhesive (e.g. silver-filledconductive epoxy) or anisotropic conductive adhesive. Other materialswell known in the art of flip chip assembly may also be used. Thestiffeners 16 extend in length above the height of the contained memoryIC chips 22 and form a seal across the top with an adhesive bond 14 tothe flexible circuit. The adhesive near the top of the module 40 may bea different material than that chosen for the attachment of thebifurcated plies of the flexible circuit. For example, the top adhesivecan be a Pressure Sensitive Adhesive (PSA) chosen to enable the moduleto be pried apart for possible access to the contained memory componentsin order to remove and replace failed devices, while the bottom adhesivecan be an electrically conductive adhesive 14′ to provide a groundingfunction as previous described. Alternatively, the top portion of thestiffeners can include an integral mechanical snap for locking thestiffeners together (not shown) or use one or more clips (not shown) topinch them together. Since the bifurcated flex circuit plies areadhesively bonded along the outer length of the module to the bottomedge, they provide a seal and flexible hinge at this location, enablingthe stiffeners to be folded out and downward to allow access to theinternal components.

Additional Symmetry & Grounding Solutions

When a chip is mounted on a flex circuit, another way to achievesymmetry is to have a flex bend on the same side of the chip in twoopposite directions as shown in FIG. 7. In addition to signal symmetry,the ground layer and signal layers can be designed to enable varioususeful functions. At high frequencies it is helpful to have the chipdesigned with large ground planes. This can be achieved by extending theflex interior ground plane layer 9 and wrapping it around the chip. Thechip under that ground plane layer is fully enclosed and therefore hassignificant EMI shielding built into it, FIG. 8.

Since the assembly shown schematically in FIG. 8 has no structuralrigidity, a stiffener 16 can be used to provide the chip(s) 22, 22′ or22″ with backing support for handling and or insertion, as shownschematically in FIG. 9. Since the stiffener can be made of metal, agrounding method is thereby formed; the electrical continuity ismaintained with the grounding plane of the stiffener. The stiffener inthis case provides several functions including: mechanical protection,handling form factor, grounding plane, and thermal dissipation (heatsink).

FIGS. 7-10 illustrate another sequence for building the inventivemodule. A double density, as shown in FIG. 10, is achieved when a mirrorimage structure is mounted on the back of the assembly shown in FIG. 9.The bifurcated flex is now disposed on both ends of the modular assemblyas shown in FIG. 10. It will be appreciated that the resulting structurebuilt up as shown in FIGS. 7-10 is similar to the structure built up asshown generally in FIGS. 2 and 4-6. Thus, the steps required to buildthe inventive module may be carried out in several different sequences.

When multiple chips are used, a multi-chip module (MCM) is formed usingflex bifurcation on the bottom of the module or alternatively at thebottom and the top of the module, 10 a, FIG. 12. As shown at FIG. 10,when the metallic heat sinks are assembled together, a cavity is formedin which all the chips are able to reside. This provides a largeadvantage in that package-less chips can be used which saves cost acrossmultiple chips. The cost saving per chip will be multiplied by thenumber of chip placements in the module.

The connector pads 18′ can be located on the tip of the flex arch, FIG.12. This enables the direct connection between the module and the motherboard.

Since the above-described modules have connections from the top as wellas from the bottom, they offer the possibility of module stacking. Itwill be appreciated that the electrical connection can be fashioned indifferent ways while remaining within the spirit of the presentinvention.

Lateral stacking and vertical stacking are both made possible by thepresent invention. FIGS. 13 and 14 illustrate various stacking methods.FIG. 13 illustrates how two modules similar to FIG. 10 can be stackedlaterally or side-by-side by electrically joining the inter-modularconnection pads 18″. FIGS. 15A-C further illustrate several differentvertical stacking configurations. FIG. 15A is similar to the module ofFIG. 12 and shows contacts 18′ at the tips of the bends of the flexelectrically connected between the top and bottom stacked modules. Thebottom module of FIG. 14 b is generally similar to the module in FIG. 10and shows a male/female stacking configuration in which the top maleedge card contact pads 18 on the double bifurcated center flex 10 a ofthe bottom module are inserted into a upper female receptacle with abifurcated center flex with edge card contact pads 18 on an interiorcavity formed in the base. As seen in FIG. 15C, the top module may bewider than the bottom module. In this case, while the bottom module,similar to FIG. 10, is cooled from the external surfaces, the topmodule, with a generally U-shaped stiffener 16′, by virtue of its openconfiguration can be cooled from the inside through the cold aircirculation.

In yet another embodiment of this invention, the stiffeners may comprisemetal heat dissipating plates 16 that are surrounded and bonded to aframe consisting of injection molded plastic 70, FIGS. 16A, 16B, 24-28,30-32, 35-38. The plastic frame 70 can include small locating pins orposts 81 in one side of the module and mating sockets in the other sideof the module to engage with alignment holes in the flexible circuit.This would enable the flexible circuit pads 18 to be accuratelypositioned and referenced with respect to other molded-in keying ororientation features on the frame. Molded features within the plasticframe would also enable the module to engage with clasps or lockingmechanisms or extraction mechanisms design as parts of the module'smating socket (not shown).

As shown FIGS. 16A and 16B, multiple thin spacers 120 (ribs or posts)are fashioned on the metal or composite heat spreaders 16 and positionedbetween groups of semiconductor chips or individual chips to preventpotential damage of the chips should an external pressure or force beapplied against the outer surfaces of the two halves of the clam shells16, pushing the chips toward each other.

Multi-Density Connections

The minimal thickness module design using bifurcated flex as describedherein can be implemented in various configurations. As can be seen fromFIG. 18 the bifurcation of flex 11 can be done for modules that arestaggered in the z (height) axis to produce a multi-points connection.These staggered bifurcated 122 modules can be configured in mirror imagefor increased density as shown in FIG. 19. The same concept can be usedfor three or more modules connection as suitable for higher densities. Athree-module connection is shown in FIG. 20. A multi point and highdensity connection can be implemented using a special mating connectoras shown schematically in FIG. 21 (about to engage) and FIG. 22 (fullyengaged).

Another significant benefit of the inventive bifurcated flex design isthe ability to add passives 35′ in the fold of the plies to achievecapacitive and resistive functions in addition to the ground connectionenabled by contacting the metal clam shell as illustrated in thecross-section view FIG. 23. The passive devices may include chipresistors, chip capacitors, thin- or thick-film resistors or capacitors,inductors, thermistors, and varistors. Skilled artisans will appreciate,therefore, that birfurcation of the flex as taught herein allowsadditional design elements that are otherwise not easy to integrate.

Once the embedded passives 35′ as well as the metallized layers arebonded to the metal enclosure, a laser trimming and splitting operationmay be used to split and create the various lateral connection pads.When separated by splitting (not shown), the contact pads are able toindependently operate in achieving and maintaining compliant contactwith the mating contacts on either a socket or adjacent module.Alternatively, the contact pads may conform over shaped spring contacts(not shown) beneath that are operable by electrical/mechanical forces(e.g. piezo-electrical or mechanical effect) or simple bumped structuresas previously described.

Description of the Exterior Shell

A thermally conductive outer or exterior shell 16, or U-shaped stiffener16′, serves several purposes. It mechanically encloses and protects thedevices within an interior cavity to prevent direct contact with thefragile chips during handling. It also provides an electrostatic andelectromagnetic shield when properly grounded to prevent possible damageto the contained ICs from electrostatic discharge and/or preventspotential electromagnetic interference with other closely spacedadjacent components. When properly connected in thermal communicationwith the enclosed devices the outer shell also provides a means forconducting heat from the operating chips to the external surfaces of theouter shell where the heat can be transferred by either static or forcedconvection air flow. Alternatively, the exterior shell can form a liquidand/or gas tight enclosure through which a circulating fluid or gas canbe introduced and removed to provide direct cooling of the deviceswithin the module's cavity.

The exterior shell preferably comprises a thin metal such as aluminum orcopper, or metal alloy such as steel or Kovar that is either machined orstamp formed. Many ceramic materials can also be substituted, providedthey have sufficient strength and thermal conductivity. Metal can beeasily stamped, pressed or cast into a variety of shapes to improverigidity, resistance to bend or twist, and thermal dissipation. Inaddition, metal fittings or ports can be fashioned or added to theexterior shell to enable the introduction and removal of circulatingliquids or gases within an interior cavity of the module.

When adapted for memory module applications, the exterior shell wouldpreferably have a generally rectilinear shape. If prepackaged memorycomponents (e.g. FBGA, μBGA, CSP, etc.) are used, the combined thicknessof these components, when mounted on both front and backside surfaces ofa central flex circuit or thin PWB, can exceed the JEDEC (JointElectronic Device Engineering Council) specification for the modulesubstrate thickness (1.27±0.10 mm) as measured across the width of theedge finger contacts (pad-to-pad thickness). In this instance theexterior shell can have flanges surrounding a pocket shaped area thatbulges out slightly to allow for the protruding DRAM packages, FIGS.12B-E, 16A-B, 47A-B, 49, 52. If, on the other hand, bare silicon memorychips are mounted onto the central flex circuit or thin PWB, theircombined thickness would be less than the specified module thickness andthe exterior shell can essentially consist of a flat metal or ceramicplate.

The exterior shell is intended to be an integral part of the module asdistinguished from the current practice of adding separate metal heatspreaders on top of traditional surface mounted DRAM components andusing metal clips to hold them in place. This current practice adds tothe overall mass and thickness of the module (≧6.5 mm) which impedes theflow or circulation of cooling air between adjacent modules that areclosely spaced together. Using an integral heat spreading exterior shellin combination with a thin substrate enables the module thickness to besignificantly reduced (≦3.5 mm) and helps achieve a minimum mass. Sinceno components are exposed on the exterior surfaces, the module'srobustness is greatly improved, potentially enabling robotic insertionof the modules into their respective DIMM sockets.

In many applications of the invention, it is contemplated that twoseparate exterior shells 16 or metal frames 82 are bonded to both sidesof a laminated PWB frame 73 or rigid-flex substrate 90 and function asstiffeners to support the thin flex circuit and form a major portion ofboth front and backside surfaces of the module. However, in anotherembodiment of this invention, the exterior shells form almost the entireouter surfaces of the module and in particular the bottom edge portionof the module, FIGS. 11, 12A-E, and 57. A center flex circuit,containing a bifurcated bottom section, extends beyond the bottom edgesof the exterior shells. This bifurcated portion of the flex circuit isthen wrapped around the bottom edges of the two metal shells, which aresymmetrically joined together, FIG. 12C, to form the contact pads forapplying electrical signals and power and ground connections to and fromthe socket. These contact pads 18 would consist of etched metal (e.g.copper) or sputter deposited traces (e.g. tungsten-copper) that arecoated with a sequence of tarnish and scratch resistant plated metals(e.g. nickel and gold) intended for mating with the pins of existingDIMM type sockets. Alternatively, the extreme bottom edges of the foldedflex circuit sections can incorporate electroplated copper-nickel-goldbumps or other electrically conductive bump material (e.g. silver-filledconductive epoxy bumps) intended for direct compression mating withelectrical pads or annular openings in the motherboard, thus eliminatingthe need for sockets with electrical pins, FIGS. 13, 43-45, 53-54. Inthis embodiment an elastomeric material 96 such as a silicone rubberstrip may be advantageously included on the backside surface of the flexcircuit opposite the electro-plated bumps in order to enable bettercompliant mechanical contact between the electrically conductive bumpson the flex and mother board contacts, FIG. 44.

In another embodiment of this invention the two separate exterior shells16 can be joined with metal tabs 125 across the top edges of the modulein a manner that enables the two shells to be folded about a center axisbetween the two halves, FIGS. 47B and 48. In this manner the exteriorshells are pre-joined along one of their respective edges and can befolded together like a clam shell with the tabs acting as a hinge. Thecenter flex circuit or thin PWB would be sandwiched within the center ofthe folded exterior shells and properly oriented by means of small postsincorporated within the interior edges or ends of the shells that engagewith similar sized and oriented holes in the flex circuit or thin PWB.In this manner the module can be subsequently unfolded and reopened forrepair if necessary.

In a similar manner, represented in another embodiment, FIG. 48, asingle flexible circuit can be wrapped from the top of one side, downand around the bottom of an “internal” heat spreader or cooling core orchamber 124, and back up the opposite surface and to the top edge on theside opposite from the previous side. The chips may then be mounted onboth sides of the flexible circuit and protected by a foldable metalenclosure as discussed above.

Description of Supporting Hollow Frames

In some embodiments, FIGS. 16-22, 24-49, 50-55, the module can alsoinclude supporting hollow molded plastic frames 70 or PWB frames 73 thatsandwich both sides of the central flex circuit or thin PWB. Thesehollow frames may be adhesively attached to both sides of a flexiblecircuit or thin PWB, which preferably occupies a center plane of theassembly and on which the components are mounted. The hollow frames alsoact as front and backside spacers that create a nested cavity for thesecomponents. They also provide a mounting surface for the flanges of thetwo separate exterior metal shells 16, FIGS. 16, 24-28, 30-49, and 52,which when attached completely enclose the cavities formed by the hollowframes.

In one embodiment these supporting frame members 70 comprise two thinmirror-imaged PWB boards that include standard etched-copper metal padswith Ni—Au plated surfaces arrayed along the bottom exterior edges andintended to mate with current DIMM sockets, FIGS. 25, 29, 32-34, 38-48,50, 52-55. The combined thicknesses of the center flex circuit or thinPWB, the adhesive used for bonding the supporting frame members, andboth halves of the frame members themselves, could be constructed toconform to the current JEDEC specification for the substrate thicknessof the edge finger contacts (1.27±0.10 mm). The etched-copper pads canbe electrically joined to traces or pads on the flex circuit or thin PWBin a number of different ways, as illustrated in FIGS. 23-45.

In another embodiment the hollow frames 70 consist of plastic moldedpieces that can incorporate a variety of different external and/orinternal electrical contacts as shown generally in FIGS. 24, 26-28. Thecontacts are formed onto the molded plastic as either thin metal padsthat are embossed or thermal stamped onto the surface or emplaced aswrap-around metal edge clips, and can include drilled and plated viaholes or formed with molded slots for metal inserts or holes for metalpins.

According to yet another embodiment the hollow frames 70 may compriseplastic molded pieces without any external contact pads, as illustratedgenerally in FIGS. 15B and 16-22. In this instance the frames 70 can bemated with a bifurcated center flex circuit having contact pads 11 thatextend beyond the bottom edges of the molded frames, as illustrated inFIG. 16A, and which are wrapped in opposite directions around the bottomedges of the two frames 70 in a manner as previously described.

The exterior shells 16 can be attached to the hollow frames 70 byseveral means. In one instance a pressure sensitive adhesive (PSA)perform is pre-applied to either the flanged edges of the exteriorshells or the edges of the hollow frames before the pieces are bondedtogether. Alternatively, the adhesive may be screen printed onto eithersurface. Alternatively the interior mating surfaces of both flange areascan consist of a metal or metal alloy over the plastic that would enablethe parts to be bonded with a fusible solder perform or laser welding orultra sonic welding. The attachment of the exterior shells to the hollowframes can be performed either before the hollow frames are bonded tothe center flex circuit or thin PWB or after.

Description of the Interior Interconnecting Substrate

The interconnecting substrate preferably comprises a thin multilayerflexible circuit or thin multilayer rigid printed wiring board (PWB)that is approximately 0.008″ (200 μm) thick. The interconnect substratewould preferably occupy a center plane within a symmetrically shapedmodule such that individual integrated devices 22, 22′, 22″ and passivecomponents 35 are mounted on both front and backside surfaces of thesubstrate and, if needed, electrically connected across the thickness ofthe substrate. The interconnecting substrate can also incorporatepassive components that are integrated 35′ within or on the surface ofthe substrate itself consisting of thin or thick film resistors or thinmetal capacitive plates layered above each other and separated withdielectric films. Integral passive components of this type are wellknown within the industry.

The interconnecting substrate material may consist of polyimide, liquidcrystal polymer (LCP), polyester film and many other materialswell-known within the industry. In some applications for the inventionsdescribed herein, the flex material may exhibit opto-electric propertiesthat enable optical signals to be transmitted between the integrateddevices mounted within the module. The flex circuit material may eitherbe optically transparent to the optical signals, modified with punchedor etched holes to allow optical signals to be transmitted through thethickness of the flex material, or contain special coatings withrefractive properties suitable for fashioning waveguides across thesurface of the flex circuit.

Description of the Integrated Circuit Devices

The Integrated Circuit (IC) devices 22, 22′, 22″ contained within theinterior portion of the module can include DRAM, SRAM, PSRAM, Flash,MRAM, and other newer memory type devices currently under development,or logic devices such as Registers, Advanced Memory Buffer (AMB), PhaseLocked Loop (PLL), Serial Presence Detect (SPD) and other similardevices intended to function as driver, buffer, control and interfacedevices. In addition microprocessor devices can be incorporated withinthe module and inter-connected with memory chips placed in adjacentlocations. In the future IC devices with optical emitters and detectorsfor signal propagation can be employed.

The aforementioned IC devices are preferably attached to theInterconnecting Substrate using flip chip or direct chip attach (DCA)technology. In this manner the IC devices occupy the smallest footprintarea on the substrate and are oriented with the active surface of theICs towards the substrate. The IC devices can occupy either the frontand/or backside surfaces of the interconnecting substrate.Alternatively, the IC devices can be individually packaged in moldedplastic and attached to the substrate with solder using surface mounttechnology (SMT).

Description of the Exterior Contact Pads and Means of Connection withthe Interior Interconnecting Substrate

In some embodiments, FIGS. 32-34, 38-48, 50, 52-55, of the presentinvention the interconnecting substrate is sandwiched between two thinand hollow PWB frames 73. These frames may have conventional copper cladcontact pads that conform to JEDEC industry standards for DIMM modules.The combined thicknesses of the interconnecting substrate, sandwichedand bonded between two hollow PWB frames, the adhesive materials used,and the copper clad pads, may be adjusted to equal the proper pad-to-padthickness (1.27 mm) across the bottom edge of the module for propermating with existing DIMM sockets. The PWB contact pads may beelectrically connected to the interior substrate in a manner typicallyused in the construction of “rigid-flex” circuit cards or by a varietyof methods familiar to those skilled in the art.

Contact pads for the module may also be incorporated within extendedportions of a flexible interconnecting substrate that is bifurcated andwrapped around the bottom edges of formed portions of the exterior metalshells, FIGS. 12B-E, or the bottom edges of hollow PWB frames thatcontain no copper clad pads, FIGS. 44, 45. These pads are alsopreferably designed to conform to existing JEDEC standards for DIMMmemory module sockets but can also be resized and relocated to theextreme bottom edge of the module in order to enable the modules to matewith new sockets specifically designed for extreme low profile mountingand/or zero insertion force (ZIF) mating, FIGS. 13, 15A, 24, 44, and 45.By placing the contacts at the extreme bottom edge, the module can beelectrically connected directly to the motherboard circuit without thenecessity of using intervening socket pins. This enables the module tobe mated to the motherboard with minimum profile height, electricaldiscontinuity and insertion force. A variety of bottom edge interfacecontacts are described elsewhere in this application for this purpose.

In an alternative embodiment, the contact pads formed on the flexiblecircuit are fashioned to enable inductive or capacitive coupling ofdigital signals from the socket or motherboard into the module.Electrodes for this purpose would be printed or etched into specificpatterns and layers of the flex circuit to couple RF energy in the formof digital signals from the socket or motherboard to signal traceswithin the flex circuit.

Description of the Means for Thermal Communication Between theIntegrated Circuit Devices and Exterior Shell

As noted earlier, a major goal of the present invention is to remove theheat generated by the contained IC devices as efficiently as possible.Therefore the module is designed to enable the shortest thermal pathbetween the backside surfaces of the contained ICs to the heat spreadingsurfaces of the exterior shell, whereby heat is subsequently dissipatedby conduction and/or convection to the surrounding exterior ambient airor cooling fluid. Since air is intrinsically a thermal insulator, it isdesirable that the thermal path between the ICs and interior surfaces ofthe metal shell be filled with a thermal interface material 26 thatexhibits properties of good thermal conductivity and elasticity forcushioning the fragile IC devices. Examples of materials suitable forthis purpose would include silicone elastomer composites, either in theform of a paste or compliant rubber-like film that is preferably filledwith silicon nitride, boron nitride, or other particulate filler withgood thermal conductive properties (e.g. diamond, copper, carbon, etc.).

Description of Module Assembly Methods

A common design feature for the exemplary modules of the invention is acentrally placed thin multilayered PWB or flexible circuit. It iscontemplated that this interior interconnecting substrate is typicallypre-assembled with surface mounted ICs and passive devices prior toattachment of the hollow frames and/or exterior metal shells.

When hollow molded plastic frames 70, metal frames 82, or PWB frames 73are employed the exterior metal shells 16 can either be pre-attached tothe hollow frames as a sub-assembly or adhesively bonded in place afterthe hollow frames are separately laminated to the interconnectingsubstrate. Alternately, the metal exterior shells 16 can be added as alast step of the assembly by sliding them over the hollow frame, asshown in FIG. 12A, and adhesively bonding them in place or with edgeclips located along the top edge of the module which apply a pinchingforce to hold the exterior shells in place (not shown). In a preferredconfiguration the exterior shells are first pre-bonded to the hollowframes such that both halves of the exterior shells are joined togetherat a centerline along the top edge of the module using stamped metaltabs 125, as illustrated schematically in FIG. 47. These metal tabs actas bendable hinges allowing the subassembly to be folded down and aroundthe interconnecting substrate. The folded subassembly sections are thenlaminated to the edges and sides of the enclosed interconnectingsubstrate using either a pressure sensitive adhesive (PSA) or athermoset adhesive such as epoxy. The final step of assembly for thisconfiguration is the electrical and/or mechanical connection of thebottom edge finger contacts.

In one embodiment the interconnecting substrate is a flexible circuitthat extends some distance beyond the lower edge of the hollowed frame.The extended flex circuit is bifurcated in a manner to enable twoseparate flex portions to be folded in opposite directions and bondedaround the outer bottom edges of the hollow frame. In this example a PSAadhesive, thermoset adhesive such as epoxy, electrically conductiveepoxy, or even solder (when joining metal to metal) may be used for thisfinal step of the assembly. Contact pads intended for mating with astandard DIMM socket are exposed and arrayed along the outside surfacesof the folded flex circuit portions. The contact pads are in turnelectrically connected through traces to the interior laminated layersof the interconnecting substrate.

In other embodiments the interconnecting substrate is either flush orextends only a short distance beyond the lower edges of the hollowed PWBframes containing DIMM-type copper clad contact pads. The extreme bottomedges of the hollowed frames contain an array of metallizedcastellations 123 (crescent shaped copper plated through holes) thatelectrically bridge across the thickness of the frames and enable theexterior PWB contact pads to be electrically connected with interiorpads arrayed on the either side of the interconnecting substrate. Themetallized castellations are electrically connected to the pads of theinterconnecting substrate using either solder or electro-platedconnections, FIGS. 41-45, 53-56.

In summary the assembly steps for non-bifurcated flex modules are asfollows:

a. Assemble all IC and passive devices to a thin multilayerinterconnecting substrate.b. Form metal exterior shells configured as two symmetrical heatspreading surfaces that are joined together along their lengths withbendable metal tabs or as two individual pieces.c. Form two symmetrical hollow frames.d. Adhesively bond the stamped metal exterior shells to the two hollowframes forming either a unified subassembly or two separatesubassemblies.e. Adhesively bond the hollow frames to an interconnecting substrate.f. Form electrical contact pads with the interconnecting substrate.

In another embodiment of the invention the hollow frames are eliminatedand the metal exterior shells are adapted for mating with a thin,flexible, and bifurcated interconnecting substrate as previous discussedabove. The bifurcated portions of the flexible interconnecting substrateare folded in opposite directions, as previously described, and bondedaround the outer bottom edges of the metal exterior shell.

In summary the assembly steps for this alternative embodiment ofbifurcated flex modules are as follows:

a. Assemble all IC and passive devices to a thin multilayerinterconnecting substrate.b. Form metal exterior shells configured as two symmetrical heatspreading surfaces that are joined together along their lengths withbendable metal tabs or as two individual pieces.c. Adhesively bond the exterior shell(s) to the interconnectingsubstrate.d. Form electrical contact pads with the interconnecting substrate.

Another embodiment, shown generally in FIG. 44, uses a single flexiblecircuit that is folded 98 upon itself and includes plated contact bumps97 or pads located at the extreme bottom edge of the module. Theflexible circuit remains centrally positioned within the thickness ofthe module, but is wrapped upon itself to form two separate circuitsthat are electrically connected together along the area of length andwidth of the folded portions 95. In essence the flexible circuit is afour-layer multilayer circuit created from a single sheet ofdouble-sided flex material by means of folding the circuit in half andconnecting pads or bumps 24 arrayed across the two inner surfacestogether. The means for providing electrical interconnections betweenthese inner layers is either solder, ACA, ICA, thermo-compressionbonding and other techniques well known within the industry. Using afolded flex circuit enables the cost of the circuit to be kept to aminimum as well as enabling the emplacement of an elastomeric material96 (e.g. silicon rubber) along the fold axis to fashion a compliantcontact means on the opposite surfaces of the folded flex portion. Sincethe cost of the flexible circuit is a function of the number of layerswithin its cross-sectional thickness and since double-sided flexiblecircuits are currently used in high volume applications, the intent ofthis embodiment is to enable the manufacture of a four-layermultilayered flexible circuit from a single sheet of double-side flexcircuit material. Alternatively, a four-layer flexible circuit, whenfolded in half and internally connected as described above, can be usedto fashion an eight-layer flexible circuit.

The folding process, for forming a multilayered flex circuit from asingle sheet of double-sided flex circuit, can be advantageouslycombined with component assembly through an SMT assembly line. Whenproperly designed, the flex circuit can have all surface mountedcomponents electrically/mechanically attached onto only the top surfaceof the double-sided flex circuit. Then using a cutting tool (e.g. laseror water-jet) one-half of the flex circuit can be cut around thecircuit's perimeter to loosen it from the surrounding flex carrier. Thisfree half can then be folded underneath itself (i.e the portion of theflex circuit that remains attached to the carrier film) such that halfof the components are on opposite surfaces of the folded flex circuit.In this configuration, the folded flex circuit is sandwiched in themiddle between surface mount components and is folded about a centeraxis parallel to the length of the circuit. The inner layers of thefolded flex circuit would be aligned and selectively bonded togetherboth electrically and mechanically to form the bifurcated ornon-bifurcated flex circuits of the inventive module embodimentsdescribed herein. This unique assembly process flow would be extremelyefficient using either inline, reel-to-reel, assembly equipment orcarrier supported transfer of the flex through a conventional SMTassembly line. The typical need to flip the substrate over for a seconddouble-sided pass through the assembly line would be eliminated by thisassembly technique, saving time and increasing SMT throughput.

The inner layers associated with this folding process may consist ofeither conductor trace patterns similar to the outer layers on which thesurface mounted components are attached, or may consist of power orground planes separated with dielectric films that include selectiveconductor paths through the thickness, FIG. 44C.

An array of contact bumps 24 or plated bumps 97 or pads 100′ along thebottom edge of the module enables the module to be connected to themotherboard without the necessity for using sockets with pin contacts.This contact means enables the module to be mounted in a verticalorientation with a minimum profile or height above the plane of themotherboard and allows the module to make direct electrical interfacewith the motherboard, thereby reducing electrical parasitics.Alternatively, the flexible circuit can include standard DIMM contacts100 arrayed along the bottom external edges of the PCB window-frames 73and include additional means for connecting contacts of the foldedflexible circuit similar to those discussed in the previous Figures.This would thereby enable the module to be interchangeable adapted foreither direct connection to the motherboard or into a standard DIMM typesocket. The bumps arrayed along the bottom edge of the folded flexcircuit may be fashioned as previously described elsewhere.

A summary of the assembly steps for an alternative embodiment of abifurcated flex module fashioned by folding a double-sided flex circuitinto a multilayered circuit is as follows:

a. Fashion a double-sided flex circuit with selective contact points onone surface and on opposite sides of a folding axis.b. Assemble all IC and passive devices to the surface opposite thecontact points.c. Fold the flexible circuit along the fold axis such that the selectivecontact points are aligned across from one another on opposite sides ofthe folding axis.d. Form electrical connections between the selective contact points thatare aligned.e. Form metal exterior shells configured as two symmetrical heatspreading surfaces that are joined together along their lengths withbendable metal tabs or as two individual pieces.f. Adhesively bond the exterior metal shells to the interconnectingsubstrate.g. Form electrical contact pads with the interconnecting substrate.

Provisions for Repairs

In each of the examples noted above the metal exterior shell can beaffixed to the hollow frame or interconnecting substrate using a PSAtype adhesive. Therefore this portion of the assembly is able to beremoved, if necessary, to enable access to the IC devices containedwithin the protected cavity for removal and replacement.

FIGS. 24 through 48 illustrate various methods by which the mating ofthin laminate PCB or a flex circuit can be made with 2 PCB frames 73. Anadditional heat sink cap or U-shaped stiffener 16′ may be added as aclip from the top as shown in FIGS. 12A and 48. The heat sink 16 or 16′in this case is made from one foldable piece of metal. Another way toassemble a thin module using one FR4 frame rather two consists of usinga single frame and bifurcating the flex to yield a module described inFIG. 49. Yet another way to maintain symmetry is to use a slit FR4 fromthe top and one-piece at the bottom. This is represented in FIG. 52. Theframe material can made of other material compatible or superior to theproperties of FR4.

In all of the illustrations, it is important to note that the modulearchitectures are compatible with monolithic chips 22″ as well asstacked chips 22′. Also all of the illustrated architectures arecompatible with a double row of chips as illustrated in FIGS. 49-52.

The connection method shown generally in FIG. 56B enables a selfalignment option since the contacts that protrude from above themotherboard are contained within the arches of the castellations.

FIG. 24 is a cross-section illustration of a centered flexible circuitsubstrate 10′, containing semiconductor chips and passive chip deviceson both surfaces, that is housed between two metal heat spreaders 16that are incorporated within a molded plastic frame 70. The moldedplastic frame includes tapered posts 81 in the bottom half that matewith corresponding recesses in the top half to enable proper placementand registration of the flexible circuit within the module forsimplified assembly of the two molded frames together. The DIMM socketcontact pads 100 on the outer surfaces of the molded plastic frames areincorporated onto both frame halves and electrically routed or connectedto pads on the interior surfaces of the frames by two possible means:(a) wrap-around pads 71 that are embossed or thermal imprinted orotherwise transferred onto the surfaces of both halves of the plasticframes in a 3-dimensional manner such that a continuous electricalconnection is made from the outer contact pads to inner pads across thebottom-edge thickness, and (b) thin metal wrap-around edge clips thatare either molded in place or inserted after the plastic frames aremolded. Electrical connections between the flexible circuit pads and theinterior pads of the wrap-around or edge clip pads are formed using oneof several means; (a) either of a pressed mechanical contact, (b)anisotropic conductive adhesive (ACA) or isotropic conductive adhesive(ICA), (c) solder, (d) or metal plating.

FIG. 25 illustrates another cross-section illustration similar in manyrespects to FIG. 24. However, the hollow frame in this embodimentconsist of thin PWB frames 73 containing standard copper laminated andetched DIMM socket contact pads near the bottom edges of the outsidesurfaces that are electrically routed to the interior surface pads usingconventional plated through holes 79. The two PWB frames are in turnover-molded with plastic rims or frames 70 that hold the metal heatspreading plates 16 to the PWB frames 73. Plastic tapered posts 81 arealso molded to the inner surface of the bottom half of the PWB frames,intended to mate with corresponding recesses drilled into the top halfof the PWB frame to enable proper placement and registration of theflexible circuit within the module and simplified assembly of the twoframes as previously mentioned. Electrical connection is establishedbetween the flexible circuit pads and the interior through-hole orvia-hole plated pads 72 by either of the means previously mentioned forFIG. 24.

FIG. 26 illustrates another cross-section embodiment similar to FIGS. 24and 25 in which electrical connections between the exterior DIMM contactpads and the interior flexible circuit pads is accomplished by means ofpress-fitted, piercing contacts 76 that are inserted into molded slotswithin the molded plastic frames. These piercing contacts wouldestablish electrical contact with the flex circuit pads by mechanicalmeans when the molded halves of the hollow frames are assembledtogether.

FIG. 27 illustrates in cross section yet another embodiment in which thecenter flexible circuit 10′ is electrically connected to the externalDIMM contact pads by means of a thin ACA layer 78. Both halves of theplastic molded frame 70 contain plated through-hole connections 79 andcontact pads 72 with integral thin film tungsten resistive heaterelements 77 located beneath the interior pads to enable the ACA to bethermally bonded between the interior contacts pad of the flex circuitand plastic molded frame following assembly. When an electrical currentis applied to designated pins of an assembly or manufacturing socket theintegral heater elements provided a source for internal heat to activatethe ACA adhesive which, together with the applied normal forces of thesocket pins, enables electrical connections for each contact pad pair tobe established.

FIG. 28 illustrates in cross section yet another embodiment in which thecenter flexible circuit 10′ is electrically connected to the externalDIMM contact pads which consist of formed or stamped metal “T” shapedcontacts 80 that are either molded in place or inserted into moldedslots within the plastic frames. These metal contacts penetrate throughthe thickness of the molded plastic and make direct contact with thepads of the flexible circuit following assembly of the frames on eitherside of the center flexible circuit substrate.

FIG. 29 illustrates in cross section shows yet another embodiment inwhich thin PWB strips 83, that contain the proper DIMM socket contacts100 are simultaneously soldered to the center flexible circuit when thememory components and passive chips are assembled onto the surfaces. Thesubstrate subassembly is then subsequently housed within a metal frame82 enclosure that captures and holds the top edges of the PWB strips.The PWB strips contain standard etched-copper and solder-coated contactpads and plated through-holes 84 that electrically and mechanically matewith the pads arrayed on the flexible circuit by means of fusible solderalloys that are reflowed at the same time as other SMT components areassembled in place. The metal frame 82 is then subsequently assembledaround the flex circuit subassembly to protect the components and tofirmly grasp and engage with the PWB strips, which are bonded togetherwith an adhesive (e.g. PSA) or low-temperature solder alloy.

FIG. 30 illustrates in cross section yet another embodiment, similar toFIG. 29, in which the separate halves of the molded plastic frame 70contain an array of pins 86 and sockets 85 along the edge thatelectrically connect with the flex circuit pads and the external DIMMcontact pads. The array of pins and sockets are either molded in placeduring fabrication of the molded frames or inserted into molded-in slotsor holes.

FIG. 31 illustrates in cross section yet another embodiment, similar toFIG. 27, in which one half of the molded plastic frame 70 contains anarray of pins 88 along the edge that are pressed fitted into platedthrough-holes 87 before being over-molded with more plastic (i.e.“buried” within the plastic frame). These pins in turn mate with pads ofthe flexible circuit and with plated through-holes 87 of the oppositehalf of the frame 70.

FIG. 32 illustrates in cross section yet another embodiment, similar toFIGS. 25 and 53, in which two thin PWB frame-halves 73 include an arrayof castellated plated through-holes 89 along an inside edge of thehollow frame to enable electrical connections between the external DIMMsocket contact pads 100 and the flexible circuit 10′ sandwiched betweenthe PWB frame-halves 73. The PWB frame sections also include over-moldedplastic rims or frames 70 that bond the metal heat spreaders 16 to thePWB frames. Solder that is pre-applied inside the castellated holes 89is reflowed during assembly of the frames to provide electrical joiningof the external contact pads with the pads 100 of the flexible circuitsubstrate.

FIG. 33 illustrates an exploded cross section view of yet anotherembodiment, in which the flexible circuit is laminated within the PWBframe halves as a “rigid-flex” circuit card or substrate 90 thatincludes conventional plated via connections between the external DIMMsocket contact pads and the internal flex circuit pads. No moldedplastic is shown, although it can be optionally added, and the metalheat spreading plates 16 are attached directly to the PWB frames usingan adhesive.

FIG. 34 is an exploded cross section view of yet another embodiment,similar to FIG. 33 except that a thin PWB 91 is substituted for aflexible circuit.

FIGS. 35 and 36 illustrate a step-wise sequence of an assembly processusing frames similar to those described earlier for FIG. 25. An extendedlength of flexible circuit bridges across both halves of thepre-assembled foldable frames 92 that are abutted edge-to-edge. Pads onthe bottom surface of the flexible circuit are electrically joined withpads on the interior surfaces of each half of the frame before foldingthe halves together into a final assembly. In this embodiment theflexible circuit also functions as a hinge about which the module halvescan be folded into their final closed configuration.

FIG. 37 shows an exploded cross section of yet another embodiment,similar to FIG. 30, in which threaded screws 94 are used to mechanicallyfasten the separate halves of the frames together. Solder bumps 93,pre-deposited onto the flexible circuit, are reflowed to provideelectrical connections between the pads of the flexible circuits and theinterior pads of the separate frames. A PSA adhesive 2 is also used tomechanically fasten the frames together.

FIG. 38 is an exploded cross-section view similar to FIG. 25 toillustrate how solder bumps 93 can be pre-deposited onto the flexiblecircuit before the separate halves of the frames are assembled together.The solder is then reflowed to complete the electrical connectionsbetween the pads of the flexible circuits and the interior pads 72 ofthe separate frames. A PSA adhesive 2 is also used to mechanicallyfasten the frames together.

FIGS. 39 and 40 illustrate an exploded view and final assembly view ofthe “rigid-flex” circuit card or substrate 90 described earlier for FIG.33.

FIGS. 41-43 illustrate a step-wise sequence of the assembly processusing frames similar to those described earlier for FIG. 33. In thisembodiment the exterior DIMM contact pads 100 are solder connected 95through castellated (partial remnants of plated through-holes) edgecontacts 123 to the pads 18 of the flexible circuit as illustrated inFIG. 53. The flexible circuit 10′ extends a short distance beyond theterminus of the castellated edge contacts 123 and provides an electricalinsulation and physical barrier between the adjacent contacts of theseparate halves of the PWB frames 73 so as to prevent direct solderbridging between these abutted contacts during final assembly. Thebottom edge contacts 100′ of the module are designed to be passedthrough a mini wave-solder fountain or into a solder pot to obtain theelectrical connections 95 between the exposed flex circuit pads 18 andPWB castellated edge contacts 123. The resultant assembly measuresapproximately 2.1 mm in total thickness.

FIG. 45 is an exploded cross-section illustration of yet anotherembodiment, similar to FIG. 44, that uses two separate flexible circuitsthat are electrically interconnect to one another as described in theprevious figure but which also uses contact bumps 24 or 97 or padslocated at the extreme bottom edge that are not backed with anelastomeric material and which are folded 98 in an outward directionthrough only a 90-degree angle.

FIG. 46 is an exploded cross-section illustration of yet anotherembodiment, similar to FIG. 44, that uses a single flexible circuitfolded 98 and interconnected upon itself, which illustrates means forelectrical connection with an array of external DIMM contact pads 100 asprevious discussed. This particular illustration details one possiblemeans of electrical interconnection between the centrally locatedflexible circuit and the DIMM contact pads 100 of the PCB window-frames73 using solder bumps or ACA adhesive film 99 and can be combined withthe additional means for direct electrical interface of the module tothe motherboard as described in FIG. 44.

FIG. 47A shows a planar view while FIGS. 47B and 48 show twocross-sectional views of embodiments that include a foldable jointedheat spreader 16 or cover plate subassembly or composite clam shell 110consisting of PWB frame 73 or molded plastic window frames 70 and joinedmetal heat spreaders. These subassemblies, as previously described,include metal tabs 125 that enable the window frames and heat spreadersto be assembled together as a single, unified subassembly with integralhinge points to allow the subassemblies to be folded in a manner thatencompasses like a clam shell the central single flexible circuit, FIG.47B, or folded flex circuit 98, FIG. 48, previously wrapped around acentral cooling core 124. As illustrated, the PWB or molded-plasticwindow frames can include recessed slots 126 along the top edges wherethe metal tabs are located to enable the tabs to be recessed flush orbelow the top edges of the window frames when folded together. Theelectrical interconnections between the single or folded flexiblecircuits and the external contacts 100 arrayed along the edges of thewindow frames have been previously described elsewhere and can beapplied to these embodiments. Likewise, central cooling cores 124compatible with these embodiments have been previously described andillustrated in earlier patent applications referenced above. In thecross-sectional view for FIG. 48, illustrated with a central coolingcore 124, the jointed heat spreaders are spaced sufficiently apart toallow the cooling core to extend some distance above the top edges ofthe folded window frames. The extended core enables and additional andseparate cooling tower 121, as illustrated, to be added to the top edgeof the exposed cooling core 124 further improving the ability of heat tobe extracted from the semiconductor devices mounted within. Thereforethis embodiment enables internal heat to be advantageously removed intwo major directions; from the most central chips into the cooling core124 and towards the cooling vanes of the top cooling tower 121, and fromthe outermost chips directly towards the heat spreaders 16 on eitherside of the module. The heat is therefore extracted from this embodimentin a bi-directional manner.

In another embodiment of this invention the central cooling core 124 isof sufficient thickness and is extended an additional distance towardsthe bottom edge sufficient to enable properly dimensioned DIMM contactspads 100 to be incorporated within the external surfaces of the foldedflexible circuit 98. Alternatively, the folded flexible circuit 98 mayincorporate bottom contact bumps or pads 24, 97 or 100 as previousdescribed for FIG. 44.

Referring now to FIGS. 49 and 52: In previous embodiments the PWB windowframe 73 consists of two separate halves that sandwich the centralflexible circuit 11 in the middle between them. However, an alternativePWB window frame 73′ is represented in FIG. 49 that comprises a singlemember with an appropriate thickness and external contact pads 100 toproperly engage with standard DIMM sockets. A bottom edge of theflexible circuit 11 is bifurcated and contains contact pads 18 disposedalong the inner surface that are electrically and mechanically attachedto pads 100 arrayed along an interior edge or recessed shelf(s) (notshown) of the window frame as illustrated in the cross-section views ofFIGS. 49 and 52. The top portion of the flexible circuit may bemechanically attached to either the exterior surface of the PWB windowframe, as illustrated in FIG. 49, or a recessed shelf routed within thewindow frame, as illustrated in FIG. 52. In FIG. 49 the flexible circuitis slightly off axis with respect to the centerline of the module, whilein FIG. 52 the recessed shelf enables the flexible circuit to bepositioned along the centerline. Though the flexible circuit terminatesnear the top of the external DIMM contact pads 100 of the window frames73′ for both FIGS. 49 and 52, the flexible circuit may also be extendedto the bottom edge of the window frames (not shown) to incorporate DIMMcontact pads within the outer surfaces of the bifurcated portions offlexible circuit 11. Alternately, the window frame may also consist ofonly the bottom portions illustrated in FIGS. 49 and 52 and use theupper flanges of the metal heat spreaders 16 to capture and hold the topedges of the flexible circuit (not shown).

FIGS. 50 and 51 are similar to FIGS. 6A and 32 and have been previouslydescribed in their respective paragraphs. They are included toillustrate examples of modules with double-rows of semiconductor devicesenclosed within the exterior metal shells 16. FIG. 52 also includes adouble-row of enclosed semiconductor devices.

FIG. 53 illustrates another embodiment in which the central flexibleinterconnecting substrate 10′ is either flush or extends a shortdistance beyond the lower edges of the hollowed PWB frames 73, which aresandwiched on both sides of the flexible substrate and which containDIMM-type copper clad contact pads 100 on the outer surfaces. Theextreme bottom edges of the hollowed frames contain an array ofmetallized castellations 123 (crescent shaped or scalloped copper platedthrough holes) that electrically bridge across the thickness of theframes and enable the exterior PWB contact pads 100 to be electricallyconnected with interior pads 18 arrayed on the either side of thecentral flexible substrate 10′. The metallized castellations areelectrically connected to the pads of the interconnecting substrateusing either solder 95 or electro-plated connections.

Referring now to FIG. 54: When the solder bumps 93 or plated bumps 97are increased in size sufficient to extend beyond the edges of themodule they may be used to establish direct electrical contact with themotherboard 127. The motherboard contacts are either surface pads orpreferably plated through holes 79 that are of sufficient diameter toenable the bumped contacts of the module to self-center within theholes. Providing bumped structures on the extreme bottom edge of themodule allows the module to rest directly against the motherboard with a“zero seating plane”, thereby reducing the module height approximately3-4 mm. This design also eliminates the necessity for socket pins tomake electrical contact with pads 100 arrayed along both bottom sides ofthe module, as is currently practiced, and eliminates the need to soldersuch pins to the motherboard. The module of this embodiment is insteadheld in compression against the pads or plated through-holes of themotherboard through use of a new “pinless” vertical contact socket, asillustrated in FIGS. 55A-C. By this means the electrical path from themotherboard to the module is significantly reduced in distance,improving the signal integrity of the module-to-motherboard interface byeliminating a source for electrical discontinuities.

FIG. 55 illustrates a new “Pinless” socket 128 designed to clamp or lockmodules similar to those described in FIGS. 41-44 and 53 in directcontact and in compression against pads or plated through-holes 79arrayed upon the motherboard 127 as shown in FIG. 54. The socketcontains locking clamps (not shown) that are press-fitted or solderedinto motherboard to mount the socket onto the motherboard and latchingmechanisms 129 for locking and holding the bottom edges of the modulesfirmly against the motherboard. By releasing the latching mechanisms themodules can be extracted from the socket guide rails that are used toorient the modules with respect to the motherboard contacts. When themodules are inserted into the guide rails and pressed firmly against thesurface of the motherboard the same latching mechanism engages with theends of the module and locks the module in a state of compressionagainst the surface of the motherboard. A metal or plastic bar locatednear the bottom center of the socket engages with a centering slot 130within the bottom edge of the module and also provides a means foraccurately locating the module with respect to the motherboard contacts.

Referring now to the cross-section view, FIG. 56A, and planar view, FIG.56B, of the bottom edge of the module of FIG. 55: An alternativemodule-to-motherboard interface makes use of very thin and low profilepins 131 within a socket to contact the copper plated walls within thescalloped or “castellated” edges 123 of the module of FIG. 53. Thisconnection method enables a self alignment option since the contacts 131that protrude from above the motherboard or from a low-profile DIMMsocket are contained within the arches of the castellations, as shown inFIG. 56B. The thin “bladed” socket pins 131 would tend to self-center atthe top of the arch within the castellations 123 and prevent the pinsfrom shifting and shorting against adjacent contacts on the edge of themodule.

FIG. 57 generally illustrates a metallic clam shell similar to FIG. 11;the two halves of which are mirror images of each other. The metallicstructure has provisions for thermal pads 26 as well as spacers 120 forpreventing the chips 22, 22′ or 22″ from crushing against each otherduring handling of the final assembly as represented in FIG. 6A or 10.

GLOSSARY

-   Adhesive 2-   Single or separate ply 4-   Bifurcated plies 4-   Two plies can be spaced apart 5-   End of adhesive coverage 6-   Outer copper patterns 7-   Copper patterns 8-   Interior copper patterns or ground plane 9-   Bifurcated Multilayer flexible circuit 10-   Bifurcated Multilayer flexible circuits on both top and bottom edges    10 a-   Non Bifurcated Multilayer Flex 10′-   Folded Non Bifurcated Multilayer Flex 10″-   Central Bifurcated multilayered flex circuit with external contact    pads 11-   Polyimide core dielectric 12-   Adhesive material-dielectric 14-   Electrically conductive adhesive 14′-   Stiffener, Heat Spreader, Exterior Metal Shell, Metallic Enclosure,    Metal Plate 16-   U-Shaped Stiffener 16′-   Solid Stiffener 16″-   Edge card contact pads 18-   Contact pads at the tip of the bend of the flex 18′-   Inter Modular Connection Pads (18 or 18′ in conjunction with    conductive adhesive) 18″-   Edge card connector 20-   Semiconductor chips or Integrated Devices 22-   Stacked Die 22′-   Bare Die 22″-   Array of bump contacts 24-   Compliant Thermal conductor pad or Interface Material 26-   Card edge connector using a bifurcated flex circuit 30-   Passives 35-   Integrated or Embedded Passives 35′-   Very thin memory module 40-   Conventional or Standard DIMM Socket or Connector 50-   Molded Plastic or Hollow Frame 70-   Wrap-around contact pads or edge clip 71-   Plated via-hole with pads 72-   PCB Frame 73-   Standard DIMM thickness PWB Frame 73′-   Layer 1 PCB 74-   Layer 2 molded plastic 75-   Piercing, press-fitted contact 76-   Resistive heater element 77-   Plated via hole 79-   Metal “T” contacts 80-   Molded tapered post 81-   Metal frame 82-   Thin PCB strips 83-   Thru-hole vias with soldered plated pads 84-   Inserted Socket 85-   Pin 86-   Buried Through Hole Via 87-   Buried Press Fitted Pin 88-   Solder in half-routed (castellated) thru-via connection 89-   Rigid Flex Circuit Card or Substrate 90-   Thin PCB 91-   Foldable Frame bridged with flex circuit 92-   Solder Bump or Ball Contacts 93-   Fastening Mechanism 94-   Solder Connection 95-   Elastomeric material 96-   Plated Bumps 97-   Folded Flex Circuit 98-   Isotropic or Anisotrpic Conductive Adhesive 99-   DIMM Socket Contact Pads 100-   Bottom Edge Contacts or Pads 100′-   Composite Clam Shell 110-   Spacer 120-   Cooling Tower 121-   Staggered Bifurcation 122-   PWB Castellated Edge Contacts 123-   Cooling Core or Chamber 124-   Metal Tab 125-   Recessed Slot 126-   Motherboard 127-   Pinless Socket 128-   Latch 129-   Centering Slot 130-   Low Profile Socket Pins 131

1. A method for making a multichip module comprising the steps of:forming a continuous dielectric sheet, said sheet having a conductivepattern on a first surface thereof; attaching a plurality ofmicroelectronic devices to said first surface so that saidmicroelectronic devices are operably coupled to said conductive pattern;applying a bonding material to a second surface of said dielectricsheet; folding said sheet so that said microelectronic devices are onthe outer surface of said folded structure; and, bonding the innersurface of said folded sheet over at least a portion of its area usingsaid bonding material.
 2. The method of claim 1 wherein said sheetfurther has a conductive pattern on a second surface thereof.
 3. Themethod of claim 2 wherein said dielectric sheet further contains viasconnecting at least a portion of said conductive patterns on said firstand second surfaces.
 4. The method of claim 2 wherein a first portion ofsaid conductive pattern on said second surface is configured to engage asecond portion of said conductive pattern on said second surface aftersaid dielectric sheet is folded.
 5. The method of claim 1 wherein saidconductive pattern included a plurality of contact pads configured toengage an external contact.
 6. The method of claim 1 wherein electricalcommunication across said inner surface is achieved by bonding the innersurface with a material selected from the group consisting of: solder,anisotropic conductive adhesive, and isotropic conductive adhesive. 7.The method of claim 1 wherein said microelectronic devices are disposedsubstantially symmetrically about the axis of said fold, so that themodule is substantially symmetrical after folding.
 8. A multichip modulecomprising: a flexible dielectric sheet having a conductive pattern on afirst surface thereof and a bonding layer on at least a portion of asecond surface thereof; a plurality of microelectronic devices disposedon said first surface so that said microelectronic devices are operablycoupled to said conductive pattern; a 180° fold about a selected axis insaid dielectric sheet so that a first portion of said second surface isbrought into contact with a second portion of said second surface andbonded thereto.
 9. The multichip module of claim 8 wherein saiddielectric sheet is configured for reel-to-reel processing.
 10. Themultichip module of claim 8 wherein at least a portion of saiddielectric sheet is not bonded, so that a bifurcated portion is formed,and contact pads are formed on said bifurcated portion.
 11. Themultichip module of claim 8 wherein said microelectronic devices areselected from the group consisting of: memory circuits, logic circuits,buffer devices, interface devices, optoelectronic devices, andmicroprocessors.
 12. The multichip module of claim 11 wherein at leastsome of said microelectronic devices are optoelectronic devices and saiddielectric sheet further contains windows configured to allowoptoelectronic communication between selected devices after saiddielectric sheet is folded.
 13. The multichip module of claim 8 whereinsaid bonding layer comprises a material selected from the groupconsisting of: solder, anisotropic conductive adhesive, and isotropicconductive adhesive.
 14. The multichip module of claim 8 furtherincluding a substantially rigid frame.
 15. The multichip module of claim8 further including a heat spreading cover.
 16. The multichip module ofclaim 15 wherein said heat spreading cover comprises a foldablestructure of sufficient size to be folded around said folded dielectricsheet and enclose said microelectronic devices.
 17. A multichip modulecomprising: a flexible dielectric sheet having a conductive pattern on afirst and a second surface thereof and a bonding layer on at least aportion of said second surface thereof; a plurality of microelectronicdevices disposed on said first surface so that said microelectronicdevices are operably coupled to said conductive pattern; a 180° foldabout a selected axis in said dielectric sheet so that a first portionof said second surface is brought into contact with a second portion ofsaid second surface and bonded thereto so that an electrical circuit iscompleted between said conductive patterns on said first and secondportions of said second surface.
 18. The multichip module of claim 17wherein said bonding layer comprises a material selected from the groupconsisting of: solder, anisotropic conductive adhesive, and isotropicconductive adhesive.
 19. The multichip module of claim 17 wherein atleast a portion of said dielectric sheet is not bonded, so that abifurcated portion is formed, and contact pads are formed on saidbifurcated portion.
 20. The multichip module of claim 17 wherein saidmicroelectronic devices are selected from the group consisting of:memory circuits, logic circuits, buffer devices, interface devices,optoelectronic devices, and microprocessors.